Cytotoxic T cells (or cytotoxic T lymphocytes (CTL), as they are also known) are one of the types of cells that develop from lymphoid stem cells and that play an important role in the immune system. T cells are antigen specific, and carry antigen specific receptors. T cells also carry receptors for major histocompatability complex (MHC) proteins, and will only function in the presence of the relevant antigen and the appropriate MHC protein. Cytotoxic T lymphocytes interact with a target cell carrying the relevant antigen (e.g. a virus protein fragment on the surface of a cell infected with a particular virus) and also carrying the appropriate MHC Class 1 protein (or human leucocyte antigen (HLA) Class 1 molecule), and the CTL release cytotoxic enzymes which act to kill the target cell.